Our invention relates to a method for the manufacture of paper and to the improved paper thus obtained. More specifically, it is an object of this invention to provide an additive which may be incorporated with the stock during the manufacture of paper, thereby causing the paper to have improved retention of pigments and other desirable properties.
Our invention comprises the addition of anionic polyacrylamide polymer groups and starch derivatives containing cationic substituent groups to paper stock or pulp.
Paper that is thinner and lighter in weight is increasingly in demand to reduce the weight of printed material that must be shipped and mailed. Fillers or pigments are added in the stock during the papermaking process, prior to the formation of the sheet, to impart opacity to the finished sheet. Some of the pigments used have a mineral source, particularly clays, calcium carbonate, talc, titanium dioxide, gypsum, and zinc pigments. In addition to their ability to enhance opacity, the pigments improve brightness, printability and smoothness.
Since conventional pigments do not have an affinity for the cellulose fibers of paper, a binder for the pigment is incorporated into the paper. Conventionally, starch has been incorporated into paper to strengthen the paper. However, the starch employed for binding and strenthening the paper does not improve the pigment retention, and in many cases actually decreases the amount of pigment retained by the paper.
The addition of the non-fibrous fillers or pigments in substantial proportion makes the paper web weaker both in wet strength during its formation and in dry strength. Of course, larger amounts of starch could be added to increase the strength of the bond of the paper but this also increases the weight.
Several derivatives of starch have been introduced in recent years which are designed to increase the retention of fillers in paper and at the same time to improve the structural properties of the paper. (U.S. Pat. Nos. 2,935,436; 2,813,093; 3,017,294; 3,151,019).
We have now discovered that the addition of a novel starch derivative, hereinafter described, to the pulp in the headbox of the Fourdrinier paper manufacturing machine results in a remarkable improvement in pigment retention in the paper, together with a concomitant increase in paper strength.